Voltage control in a number of systems with high voltage, extra high voltage transmission and/or underground high voltage cables is an important system design consideration. Absorption of capacitive vars required to control voltage can presently be achieved by (a) underexcited synchronous condensers, (b) linear reactors, or (c) static var sources with thyristor control or of the saturable type. Each of these known devices has its own advantages and disadvantages.
Specifically, synchronous condensers are expensive, unreliable, and have finite absorption capability beyond which the condenser will slip poles. Aside from the pulsation in voltage in the process of slipping, the action of a voltage regulator would provide positive feedback once the condenser locked in step on a pole of opposite polarity. While this can be circumvented by special control logic, the net effect would still be the loss of the condenser. Hence, the condenser, unlike a reactor, would have a maximum absorption capability determined by dynamic rather than short term thermal considerations.
Synchronous condensers have had application in HVDC converter stations because of the need for short circuit capacity as required for proper commutation of converter valves.
Linear reactors are not very effective in limiting overvoltages since their absorption of vars would be proportional to the voltage squared, as contrasted with saturable reactors or thyristor controlled reactors.
Static var sources, of the thyristor controlled type, produce harmonics. They can control overvoltages very effectively within the off-on range of the valves. Beyond the fully-on point, the reactor would have the same overvoltage absorption capability of the linear reactor, subject to thermal limitations. Static var sources, of the saturable type, exhibit excellent voltage control characteristics and will exhibit the same droop characteristics regardless of overvoltage. Control of overvoltage would be very effective, of course, within thermal limitations. Here, however, harmonics are a problem and, special winding arrangements or filters must be provided to absorb undersirable harmonic generation.
These disadvantages of the prior art are overcome by the present invention.
Accordingly it is an object of the present invention to provide a method and apparatus for voltage control in power system networks particularly involving high and very high voltage overhead and/or underground transmission.
It is another object to provide absorption of capacitive vars required to control high voltages without the disadvantages associated with conventional methods and devices, such as dynamic limitations on maximum absorption capability, undesirable harmonics, cost and reliability.
It is a further object to provide for voltage control in systems where the voltage control device must exhibit low values of incremental inductance such as is required for commutation of currents in converter valves.